Bug 22273: Modify Koha::ArticleRequest->store to fill created_on

Bug 22273: Modify Koha::ArticleRequest->store to fill created_on

For a new record, store should fill created_on.
The database will always update the timestamp updated_on.
Since open also calls SUPER::store, we do not need to call it twice.

Test plan:
[1] Run t/db_dependent/ArticleRequests.t
[2] In the interface, add two article requests. Change the status of one
    to PROCESSING. Check created_on and updated_on in the article_requests
    table. The changed request should have updated_on > created_on.

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
[1] Tests pass
ok 12 - New article request has created_on date set
ok 13 - New article request has updated_on date set

[2] Work as described.
No errors

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>